Romney speaks a the Univision Candidates' Forum moderated in Miami on September 19.

It has been one of the running mysteries of the 2012 campaign trail: How Mitt Romney maintains the ruddy, glowing, and occasionally changing skin tone that helps him project an image of 65-year-old vigor.

A knowledgeable source tells BuzzFeed the answer is in a bit of cosmetic technology used commonly by celebrities: spray tanning. The Republican nominee has made a habit of spray tanning before major speeches, debates, interviews, and other events that have a chance of getting wide TV coverage, the source said. He pays for the process out of pocket — sparing his campaign the expense, and the task of masking it on public campaign finance reports — and steers clear of public salons where he could be recognized. Instead, he gets misted down in the comfort of his own home or hotel suite.

The Romney campaign flatly denied that the candidate spray tans: "Not true," spokeswoman Andrea Saul said in response to an inquiry.

Romney's ever-changing complexion has been a point of public curiosity throughout the campaign, especially after his appearance at a Univision forum in September. There, his unusually dark skin tone prompted conspiracy-minded Tweeters to speculate that he had purposefully applied brown makeup in order to "look more Latino." At the time, one of the network's anchors debunked the rumor to BuzzFeed, and the makeup artist even came forward to defend Romney.

"When he walked in, I remember thinking, 'Wow this is tanner than I thought he was,' but I think he's just been outside a lot lately for his campaign," Lazz Rodriguez told Univision afterward. "It was definitely a real tan."

But Romney's darker-than-usual complexion at the forum was actually the product of an exceptionally fresh spray tan, the source said. And while the web has been rife with speculation about the candidate's coloring, no one has found it more obvious than people in the tanning industry.

"Oh, for sure he had a spray tan," Jimmy Coco, a celebrity tanner from Los Angeles, told BuzzFeed while looking at pictures of the candidate. "I just see the sort of red hue that you get from some of the spray tanning places... The ears are white. That is a giveaway. If you look at Obama, he's all one tone. A lot of times, what they do when they're giving a spray tan, they'll put a cap on and their cap will cover the ears."

Mobile tanning professionals said the service generally costs between $200 and $500, depending on how far they have to travel for house calls. (It's unclear how much Romney spends, since he is always traveling.)

Methodologies vary, but Jennifer Lee, owner of Born to Glow in Beverly Hills, said the most high-tech services equip their mobile tanners with pop-up tents and portable misters, which they bring to the clients' homes, offices, or hotel rooms.

"I pop up the tent, detemine how dark the client wants to go, and then I mix the formula and match it with their skin tone," Lee said. "I have an eye for that."

Anna Stankiewicz, owner of the New York-based spray tan company Suvara, specializes in house calls, and said she has been hired by several New York City politicians over the years — including City Council Speaker Christine Quinn, who she said was "cool about it."

Robin Levine, a City Council spokeswoman, denied that Quinn had ever been a client of Stankiewicz's. "Speaker Quinn has never been spray tanned," said Levine in a statement to BuzzFeed. But Stankiewicz reiterated that she had indeed spray tanned the Speaker about three years ago at Rita Hazan Salon in New York City.

Political clients usually only get sprayed "from the waist up" because legs stay hidden inside suit pants, while forearms are often revealed when men roll up their shirt sleeves, Stankiewicz said. She added that confidentiality agreements are common among high-profile clients.

Like most of the industry experts who spoke to BuzzFeed, Stankiewicz suspected Romney spray tanned long before she was approached for an interview, and noted that it's fairly common practice in politics. After all, she said, "It's not like Mitt Romney can go chill out on a beach right now; he needs a quick fix."

Some of the spray tanning professionals also sniffed a bit at the job done on Romney.

"The color he's choosing is totally not believable," Stankiewicz said. "It's a dead giveaway... They're clearly using way too dark a forumla for his skin tone, It's just like, 'Oh my God, he got sprayed. It's just so obvious."

Tamar Vezirian, owner of New York-based mobile airbrush tanning service Gotham Glow, said she's received e-mails from clients and friends who saw the candidate on TV and said, 'Oh, did you see Mitt Romney's spray tan? Why didn't he call you?"

"His hands were really dark, and the neck was white," Vezirian said after watching Monday night's debate. "It was very uneven, and it didn't look natural at all. You can always tell by looking at the hands and the neck. On camera, it shows up a lot more too."

She added, "I was looking at Mitt Romney the whole time and just thinking, with all that money you’d think he’d get the best spray tanner in the country."

Update: This story has been updated with comments from an aide to City Council Speaker Christine Quinn and further comment from the spray tan specialist who said she worked with Quinn.

McKay Coppins is a senior writer for the BuzzFeed News politics team, and the author of The Wilderness, about the battle over the future of the Republican Party.